1. Field of the Invention
The subject invention relates to antennas and, more particularly, to a technique for producing a guard pattern for an active antenna array.
2. Description of Related Art
A guard pattern is useful in eliminating target returns outside of the main beam of an antenna. According to conventional design, the guard pattern is designed to exceed the main gain of the antenna in the sidelobe region. Simple decision logic then rejects returns whose main is not much larger than its guard. With a mechanically scanned antenna, one creates a guard pattern by selecting an appropriate guard horn with a fixed orientation to boresight. Such a configuration is effective in all scan directions.
Establishing a guard function is more difficult with an electronically scanned antenna. Since the main beam scans independently of the plate of the antenna, a single, fixed guard horn may not perform well at scans off the mechanical boresight. Two ways around this problem are known. One may either switch strategically between several differently oriented guard horns, or one may use a single guard array which scans with the main beam. The latter single guard array approach is the subject of the invention described hereafter.
A simple single guard subarray can be formed on an active array antenna by devoting one or more elements to this function. However, one element alone cannot be scanned because scanning requires a phase slope across the antenna. Two elements together can only scan in one angular dimension. Since the single guard subarray needs to scan omnidirectionally, it would appear to require four-fold symmetry. Thus, the smallest practical single guard array consists of four elements in a square. The next largest square guard array has nine elements arranged three-by-three.
The problem with such small guard arrays is the presence of excessive nulls in the guard array pattern. In the vicinity of nulls, the guard is generally useless. The square arrays described above have entire null planes which move as the guard array is scanned. Such null planes lead to unacceptable performance.